The subject matter of the present invention relates to well logging apparatus, and, in particular, to an accurate depth determination system, using parameter estimation, for use with the well logging apparatus.
In a typical well logging scenario, a string of measurement tools is lowered on cable to the bottom of an oil well between perhaps 2 to 5 km in the earth. Geophysical data is recorded from the tool instruments as the cable is wound in at constant speed on a precision winch. The logging speed and cable depth are determined uphole with a depth wheel measurement instrument and magnetic markers on the cable. The problem, however, is that, when disposed downhole, the tool string is usually not in uniform motion, particularly for deviated holes occurring in offshore wells. The suite of measurements from the tool string are referred to a common depth using depth wheel data. However, if the tool motion is non-uniform, this depth shifting is only accurate in an average sense. The actual downhole tool position as a function of time is required to accurately depth shift the suite of sensor data to a common point. When the motion is not uniform, the depth shift applied to the various sensors on the tool string is time-dependent. Therefore, given surface depth wheel data, and downhole axial accelerometer data, an unbiased estimate of the true axial position of the logging tool string is required to fully utilize the higher resolving power (mm to cm range) of modern logging tools.
The depth estimate must be coherent over the processing window of downhole sensors, but not necessarily over the entire depth of the well. Thus over the processing window (which may be up to 10 m) as required by the tool software to estimate formation features, the distance between any two points in the processing window must be accurately determined. No claim of depth accuracy relative to the surface of the earth is made. One depth determination technique is discussed by Chan, in an article entitled "Accurate Depth Determination in Well Logging"; IEEE-Transations-on Acoustics, Speech, and Signal Processing; 32, p 42-48, 1984, the disclosure of which is incorporated by reference into this specification. Another depth determination technique is discussed by Chan in U.S. Pat. No. 4,545,242 issued Oct. 8, 1985, the disclosure of which is incorporated by reference into this specification. In Chan, no consideration is given to certain types of non-uniform motion, such as damped resonant motion known as "yo-yo", arising from oscillations of the tool on the downhole cable. Accordingly, a more accurate depth determination system, for use with downhole well logging tools, is required.